Multidisciplinary Design Optimisation of Launchers coupling the Design of the Propulsion System with the Trajectory Optimisation

In this work, a bi-level Multidisciplinary Design Optimisation was implemented on the trajectory and the performances of an hybrid propulsion system for a spaceplane mission. The test case used as a reference for dimensions and aerodynamics is the CFASTT-1, a reusable launch vehicle developed for a SSTO mission whose concept is similar to the most recent ongoing project Skylon. The engine modelisation was carried out through HyPro, a medium-fidelity modular model which employs the equations of fluid dynamics to design the main components of a hybrid engine in all its configurations, airbreathing (ejector ramjet, ramjet and scramjet) and rocket. Due to its complexity, though, HyPro is too computationally expensive for the MDO process, so Surrogate Models were used instead. Once set the range of work of the four configurations, HyPro was used to create a database for each engine, inclusive of significative and uniformly distributed values of Mach, altitude and the sensitive areas, i.e. those cross sectional areas of the modules which most affect the thrust and mass flow output in each engine mode. These databases were the training sets for the Surrogate Models: neural networks specifically trained on the HyPro models data but much lighter and more suitable for a process involving multiple iterations. The main objective of the MDO process is to optimise the propulsion system for a given mission profile: an inner loop solves the control problem and finds the optimal trajectory, an outer loop optimises the engine performances, i.e. its thrust, mass flow, and volume (and, as a consequence, its weight). The target driving the whole optimisation process is the objective function, which can be modified according to the purposes of the design process. In this specific case, the goal was to maximise the payload of the mission, thus finding the optimum of weight and fuel consumption. This process was tested on a typical SSTO mission profile for access to space in low orbit.